The last issue I had with all of my PVC-Voigt-Pipes: resonance of the horn. I read about it, I knew about it... and now, I was bugged by it too.
The solution is very simple: a tripe of cleaning cloth, just below the joint of the driver unit to the horn. The effectiveness of which can be tested by slapping a flat hand on either end of the horn. Without the damping cleaning cloth sound would resonate creating some BOONNNNNG BOONNNNG. With the cloth inserted, this changes to BooB BooB.
BTW, this was effective with the designs I published on this blog.